Atmos. Chem. Phys., 10, 2867–2877, 2010
www.atmos-chem-phys.net/10/2867/2010/
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
The organic fraction of bubble-generated, accumulation mode Sea
Spray Aerosol (SSA)
R. L. Modini, B. Harris, and Z. D. Ristovski
ILAQH, Queensland University of Technology, P.O. Box 4233, Brisbane QLD, 4001, Australia
Received: 14 September 2009 – Published in Atmos. Chem. Phys. Discuss.: 12 October 2009
Revised: 20 January 2010 – Accepted: 22 March 2010 – Published: 26 March 2010
Abstract. Recent studies have detected a dominant accumulation mode (∼100 nm) in the Sea Spray Aerosol
(SSA) number distribution. There is evidence to suggest
that particles in this mode are composed primarily of organics. To investigate this hypothesis we conducted experiments on NaCl, artificial SSA and natural SSA particles with a Volatility-Hygroscopicity-Tandem-DifferentialMobility-Analyser (VH-TDMA). NaCl particles were atomiser generated and a bubble generator was constructed to
produce artificial and natural SSA particles. Natural seawater samples for use in the bubble generator were collected
from biologically active, terrestrially-affected coastal water
in Moreton Bay, Australia. Differences in the VH-TDMAmeasured volatility curves of artificial and natural SSA particles were used to investigate and quantify the organic fraction
of natural SSA particles. Hygroscopic Growth Factor (HGF)
data, also obtained by the VH-TDMA, were used to confirm
the conclusions drawn from the volatility data. Both datasets
indicated that the organic fraction of our natural SSA particles evaporated in the VH-TDMA over the temperature range
170–200 ◦ C. The organic volume fraction for 71–77 nm natural SSA particles was 8±6%. Organic volume fraction did
not vary significantly with varying water residence time (40 s
to 24 h) in the bubble generator or SSA particle diameter in
the range 38–173 nm. At room temperature we measured
shape- and Kelvin-corrected HGF at 90% RH of 2.46±0.02
for NaCl, 2.35±0.02 for artifical SSA and 2.26±0.02 for natural SSA particles. Overall, these results suggest that the
natural accumulation mode SSA particles produced in these
experiments contained only a minor organic fraction, which
had little effect on hygroscopic growth. Our measurement
Correspondence to: Z. D. Ristovski
([email protected])
of 8±6% is an order of magnitude below two previous measurements of the organic fraction in SSA particles of comparable sizes. We stress that our results were obtained using coastal seawater and they can’t necessarily be applied on
a regional or global ocean scale. Nevertheless, considering
the order of magnitude discrepancy between this and previous studies, further research with independent measurement
techniques and a variety of different seawaters is required to
better quantify how much organic material is present in accumulation mode SSA.
1
Introduction
Sea Spray Aerosol (SSA) is generated when air bubbles rise
to the ocean surface and burst or when seawater droplets
are torn from the crests of waves. These seemingly simple processes create the largest mass emission flux to the
atmosphere of all aerosol types (Andreae and Rosenfield,
2008). SSA particles vary in size over 5 decades from tens
of nanometres to hundreds of micrometres. Large supermicrometre SSA particles account for the majority of sea
spray mass in the atmosphere (>95%). However it is the submicrometre SSA particles that are by far the most numerous.
In particular recent laboratory and field measurements have
consistently detected a dominant mode in the SSA number
distribution centred at 100 nm dry diameter (Clarke et al.,
2006; Martensson et al., 2003; O’Dowd and Smith, 1993;
Sellegri et al., 2006; Tyree et al., 2007). In this study we
will refer to this dominant mode as the SSA accumulation
mode even though it extends to particle sizes traditionally
placed in the Aitken mode (20–100 nm). The SSA accumulation mode is climatically very important because it means
SSA can potentially account for a significant proportion of
Published by Copernicus Publications on behalf of the European Geosciences Union.
2868
cloud condensation nuclei (CCN) in the remote marine environment, particularly under high wind conditions (Clarke
et al., 2006; O’Dowd and Smith, 1997; Pierce and Adams,
2006).
Bursting bubbles produce SSA in the form of film drops
and jet drops. The SSA accumulation mode most likely originates from film drops. These are generated when fragments
of bubble film (cap) are ejected into the air as a bubble bursts.
Secondary droplets created when some of these fragments
collide with the air-water surface may also contribute to the
SSA accumulation mode (Spiel, 1998). Milliseconds after
a bubble burst (Spiel, 1995) jet drops are generated from the
break-up of the upward moving jet column caused by the
collapse of the bubble cavity. Jet drops are in the supermicrometre size range: they are roughly one tenth the size
of their parent bubbles (Blanchard, 1989).
The composition of SSA is surprisingly complex. Seawater contains a range of inorganic salts (see Sect. 2.2) which
all exist in SSA. In addition SSA contains an organic fraction
which is derived from on or near the ocean surface. The existence of an organic fraction in SSA was detected many years
ago (Blanchard, 1964). A number of studies found that the
concentration of organic carbon (Gershey, 1983; Hoffman
and Duce, 1976) and bacteria (e.g. see Blanchard, 1989 and
references therein) in bulk SSA is enriched hundreds of time
relative to corresponding concentrations in source water. Enrichment of organic matter in SSA occurs because it is generated from bubbles bursting in an enriched layer of chemical and biological material on seawater surfaces known as
the sea-surface micro-layer (e.g. Liss and Duce, 1997). Organic material, and in particular surface-active organic material, becomes concentrated at the sea-surface micro-layer by
factors of up to 10 compared to sub-surface waters (Hunter,
1997) due to processes such as diffusion, turbulent mixing
and scavenging and transport by rising air bubbles.
Decades ago it was hypothesised that the organic fraction
of SSA may increase with decreasing particle size (Barker
et al., 1972; Hoffman and Duce, 1974). Three recent studies have examined this hypothesis. Oppo et al. (1999) constructed a simple model that predicted the surfactant organic
fraction of SSA droplets will increase hyperbolic-like with
decreasing droplet size. The model rests on the assumption
that SSA droplets produced from the rough sea surface contain condensed, saturated films of surfactant material of constant thickness (independent of droplet size).
Experimentally, detailed measurements of the sizeresolved organic fraction of SSA produced by flowing natural seawaters through bubble generators were conducted by
Facchini et al. (2008) and Keene et al. (2007). A bubble generator mimics the bubble bursting process on ocean surfaces
to generate nascent SSA isolated from other aerosol types.
Both of these studies employed size-resolved impactor sampling and subsequent chemical analysis to show that SSA organic fraction increased with decreasing particle size in their
experiments. Both studies measured an organic mass fracAtmos. Chem. Phys., 10, 2867–2877, 2010
R. L. Modini et al.: Organics in Sea Spray Aerosol
tion of ∼80% for the lowest stages of their impactor samples
corresponding to aerodynamic diameters of 130 nm (GMD,
Keene et al., 2007) and 125–250 nm (Facchini et al., 2008).
However Keene et al. (2007) only measured soluble organics, and Facchini et al. (2008) measured both soluble and insoluble organics and found insoluble components dominate
(∼94% of organics). While both studies had different operative definitions of solubility, this consideration still implies
that the results of these separate experiments do not agree
as well as they first appear to. Nevertheless, based on these
studies it is currently expected that the accumulation mode
of SSA produced from biologically active seawater consists
of particles that are predominantly organic. Big and Leck
(2008) go even further to suggest that the particles comprising the SSA accumulation mode (<200 nm) are actually organic fragments with no inorganic component at all.
It is important to characterise the composition of particles in the SSA accumulation mode to correctly model their
climatic influence. The organic fraction of SSA particles
will affect their size as a function of RH (Ming and Russell,
2001) and therefore their scattering potential (Randles et al.,
2004), their ability to act as CCN (Moore et al., 2008) and
also their role in atmospheric chemistry (Zhou et al., 2008).
The purpose of this study was to investigate and quantify
the organic fraction of bubble-chamber-generated accumulation mode SSA using an original, independent and on-line
method: the Volatility Hygroscopicity-Tandem Differential
Mobility Analyser (VH-TDMA). In addition the VH-TDMA
was also able to measure the hygroscopic growth factors of
SSA accumulation mode particles.
2
2.1
Experimental methods
Bubble generator
A bubble generator was constructed to mimic the production
of SSA by bursting bubbles on seawater surfaces. The generator is depicted in Fig. 1. It consisted of a 1 m long glass
cylinder (id=2.9 cm) with a fritted glass tip (SKC midget impinger; pore size 170–220 µm) inserted at the bottom. Sample water entered the bottom of the generator from a 20 L
plastic drum. The 20 L drum was placed above the generator so that gravity was the driving force of water through the
system. A tap was used to control the water flow rate. Water
exited the generator through a ¼ inch plastic tube fitted with
a valve to prevent external air entering the generator. The
vertical position of the exit tube was used to set the height of
water in the generator. In these experiments the height was
set for a bubble-rise distance of 31 cm. This corresponded
to a water volume of 200 mL. Particle-free air was bubbled
through the fritted tip at a flow rate of 100 mL min−1 to produce a steady stream of bubbles. Bubble size distribution
was not measured in these experiments. A sample outlet at
the top of the generator was used to extract SSA produced by
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R. L. Modini et al.: Organics in Sea Spray Aerosol
To VH-TDMA:
2 L/min
Make-up
air
2869
From 20 L resevoir: Qw
F itte d lo g - n o r m a l fu n c tio n
S M P S d a ta
7 0 0 0
6 0 0 0
d N /d lo g ( d p ) ( c m
-3
)
5 0 0 0
4 0 0 0
3 0 0 0
2 0 0 0
1 0 0 0
0
1m
0.31 m
Qw 0.1 L/min
Fig. 1. A schematic diagram of the bubble generator used to produce sea spray aerosol (SSA). Qw is the water flow rate through the
generator which was varied between experiments.
bursting bubbles at 2 L min−1 for VH-TDMA analysis. An
inlet tube led from the top of the generator to just above the
air-water interface to allow time for particle-free make-up air
to mix with SSA before being sampled by the VH-TDMA.
All experiments were conducted at room temperature (25 ◦ C)
and SSA was dried (<10% RH) before it entered the VHTDMA. The dry size distribution (9–379 nm) of SSA produced in our generator consisted of a dominant accumulation
mode centred at ∼80 nm (Fig. 2), which compares well with
other bubble-generated SSA size distributions (Martensson
et al., 2003; Sellegri et al., 2006; Tyree et al., 2007).
2.2
Sample water
Experiments were conducted with two main types of water in the bubble generator: artificial sea salt solution (artificial SW) and natural seawater (natural SW). In addition the results were compared to VH-TDMA measurements
of NaCl particles generated from an atomised solution of
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1 0
1 0 0
S iz e ( n m )
Fig. 2. An SMPS size distribution (9–379 nm) of dry SSA produced
from natural seawater in our bubble generator. Grey rectangles indicate particle sizes that were selected for VH-TDMA analysis.
NaCl in ultra-pure deionised water. Artificial sea salt solution was generated by dissolving analytical grade sodium
chloride (NaCl), magnesium chloride (MgCl2 ), sodium sulphate (Na2 SO4 ), calcium chloride (CaCl2 ), potassium sulphate (K2 SO4 ), sodium bromide (NaBr) and potassium nitrate (KNO3 ) in ultra-pure deionised water. Two artificial
salt solutions were prepared with varying ionic composition
(Seinfeld and Pandis, 2006, p. 444; Niedermeier et al., 2008,
Atlantic Ocean sample).
Natural SW was collected at high tide on 28 January 2009
from Redcliffe Jetty, which extends 200 m into the northwestern section of Moreton Bay on the east coast of Australia. Two minor river systems lie 11 km to the northwest (Caboolture River) and 8 km to the south-west (Pine
River) of the sampling site. As such the sampling site
is subject to significant terrestrial run-off. The salinity of
the collected samples was 31.8 g L−1 , measured via electrical conductivity (Eaton et al., 2005). The organic fraction or biological activity of our natural SW samples was
not measured. However, monthly chlorophyll-a (chl-a) measurements (absorbance spectroscopy) at three sampling sites
within a 4 km radius of the sampling point were provided by
the South East Queensland Healthy Waterways Partnership
(www.healthywaterways.org). Chl-a values at the three sites
on 6–7 January 2009 varied between 1.47–3.14 mg m−3 . On
5 or 19 February 2009 chl-a values at the three sites were
in the range 1.29–3.21 mg m−3 . Therefore it is reasonable to
assume that the biological activity of our natural SW samples
was quite high. Dissolved organic carbon (DOC) content can
vary widely in Moreton Bay depending, amongst other factors, on terrestrial run-off. Approximately 14 km north of
Redcliffe Jetty in Moreton Bay, Albert et al. (2005) measured
DOC concentrations of up to 50 mg L−1 during a wet period
Atmos. Chem. Phys., 10, 2867–2877, 2010
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R. L. Modini et al.: Organics in Sea Spray Aerosol
(large run-off) in 2003. In contrast DOC concentrations were
only 10–20 mg L−1 during a dry period (small run-off) in
2002. Our sample collection was conducted in a relatively
dry period (average monthly rainfall was low and similar to
the average monthly rainfall during the Albert et al. measurements). Therefore we assume that the DOC content of our
natural seawater was less than 20 mg L−1 . Natural SW samples were refrigerated in the dark and used within 2 weeks of
the collection date. They were brought to room temperature
and thoroughly stirred before bubbling experiments began.
2.3
Volatility Hygroscopicity-Tandem Differential
Mobility Analyser (VH-TDMA)
The Volatility Hygroscopicity-Tandem Differential Mobility Analyser (VH-TDMA) has been described in detail elsewhere (Fletcher et al., 2007; Johnson et al., 2004; Modini
et al., 2009) and will only be discussed very briefly here.
The VH-TDMA was used to measure the average diameter
of initially monodisperse SSA particles as they were heated
in a thermodenuder (residence time=0.3 s) from ambient to
583 ◦ C in temperature increments of 15–60 ◦ C. Even after
particle shrinkage occurred at higher temperatures the particles retained a monodisperse distribution. This means the
average diameters before and after volatilisation could be
used to calculate average volume fraction of SSA remaining (V/Vo). Volatility curves of different particle types were
constructed by plotting V/Vo versus volatilisation temperature.
In addition, the VH-TDMA simultaneously measured the
Hygroscopic Growth Factor at 90% RH (HGF90%) of the
volatilised particles at each temperature. HGF90% measurements of non-spherical particles taken with a (V)H-TDMA
should be corrected for shape effects so they can be compared with independent measurements and theoretical predictions. The non-sphericity of dry NaCl particles is well
described. For the range of NaCl particle sizes investigated
in this study (65–98 nm) we applied a size-dependent shape
correction factor that varied from 1.213–1.199 (Biskos et al.,
2006). There is evidence to suggest that natural and artificial SSA particles are also non-spherical in shape and can
be described with the same size-dependent shape correction
factor as NaCl (Niedermeier et al., 2008; Wise et al., 2009).
Therefore we also applied the NaCl shape correction factor
to natural and artificial SSA particles in this study. To remove
the influence of the Kelvin Effect on the HGF90% measurements taken at different sizes they were converted to bulk
HGF90% values (i.e. where aw=RH=0.9) using a constant
single parameter representation of hygroscopic growth (Petters and Kreidenweis, 2007). The bulk HGF90% values are
reported here. All VH-TDMA data were inverted using the
TDMAinv algorithm (Gysel et al., 2009). Assuming a DMA
sizing accuracy of ±1% and RH uncertainty of ±0.6% the
theoretical uncertainty (95% confidence level) is ±3% for
V/Vo and ±3% for HGF90%.
Atmos. Chem. Phys., 10, 2867–2877, 2010
Difference in the VH-TDMA volatility curves of natural
and artificial SSA were used to investigate, and then quantify, the organic fraction of natural accumulation mode SSA.
This approach is based on the assumptions that 1) natural
SSA potentially contains a seawater-derived organic fraction
that is not present in artificial SSA, 2) this organic fraction is
more volatile than the inorganic fraction of SSA, 3) the inorganic composition of artificial and natural SWs used in these
experiments is very similar, and 4) any organic impurities
present in the artificial SSA were also present in the natural
SSA. The third assumption was tested by using two types of
artificial sea salt solution with varying inorganic composition
so we could judge whether small differences in the inorganic
composition of artificial SSA translated into measurable differences in the VH-TDMA volatility curves. The fourth assumption is considered a reasonable one because the precleaning process of the bubble generator was constant for all
experiments and artificial SW was prepared with ultra-pure
deionised water.
The advantages of using the VH-TDMA to measure the
organic fraction of accumulation mode SSA are that only relatively small concentrations of particles (∼100 cm−3 ) are required for the analysis, total scan time is only 1–2 h and the
lower size limit is 10 nm. Therefore only a small bubble generator and sample of water are required (see Table 1), which
reduces the chances of organic contamination. In addition,
at RHs above the deliquescence point of SSA (75%) organic
components will decrease the hygroscopic growth factor of
SSA (Ming and Russell, 2001). This means the HGF90%
measurements taken by the VH-TDMA can be used to confirm the conclusions drawn from the volatility measurements.
2.4
Experimental conditions
Table 1 lists the experiments we performed and values of
important parameters compared to other studies that have
utilised bubble generators. We chose to investigate whether
the organic fraction of accumulation mode SSA particles depended on sample water flow rate through the bubble generator and SSA particle diameter. Water flow rate is important because if it is too low relative to the bubble flow rate
(i.e. the organic source is less than the sink) then the organic
content of sample water in the bubble generator could be depleted over time. The maximum water flow rate we used
was 0.3 L min−1 . This value was large enough to ensure
that the water:air flow ratio was higher and water residence
time lower then studies where large concentrations of organics have been detected in the aerosol phase. At the other
extreme we performed one experiment with static water in
the bubble generator that was left to bubble for 24 h before
a VH-TDMA scan was conducted.
The influence of particle size on organic fraction was investigated because, as stated above, previous studies have
shown that the organic fraction of SSA increases with decreasing particle size (Facchini et al., 2008; Keene et al.,
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R. L. Modini et al.: Organics in Sea Spray Aerosol
2871
Table 1. Experimental parameters.
Experiment
Sample
water
Aerosol
generation
method
Particle
diameter
[nm]
Water volume
in bubbler
[L]
Water
flow rate
[Lmin−1 ]
Bubbling
rate
[Lmin−1 ]
Water:air
flow ratio
Water
residence
time [s]
NaCl1
NaCl solution
Atomiser
71
NaCl2
NaCl solution
Atomiser
98
A1
Artificial SWa,b
Bubbler
71
0.2
0
0.1
0
∞
A2
Artificial SWb
Bubbler
78
0.2
0
0.1
0
∞
A3
Artificial SWc
Bubbler
98
0.2
0
0.1
0
∞
N71
0
Natural SW
Bubbler
71
0.2
0
0.1
0
∞
N77
0.03
Natural SW
Bubbler
77
0.2
0.03
0.1
0.3
400
N71
0.08
Natural SW
Bubbler
71
0.2
0.08
0.1
0.8
150
N71
0.3
Natural SW
Bubbler
71
0.2
0.3
0.1
3
40
N38
0.08
Natural SW
Bubbler
38
0.2
0.08
0.1
0.8
150
N173
0.08
Natural SW
Bubbler
173
0.2
0.08
0.1
0.8
150
Keene
et al. (2007)
Facchini
et al. (2008)
Gershey
(1983)
Hoffman and
Duce (1976)
Natural SW
Bubbler
–
42
4
5
0.8
630
Natural SW
Bubbler
–
100
6–7
20
0.35
857
Natural SW
Bubbler
–
19
33.7
0.119
283
34
Natural SW
Bubbler
–
–
0.2
0.05
4
a This experiment is the average of 3 almost indistinguishable repeated scans. b Ionic mass fractions according to Seinfeld and Pandis (2006),
page 444. c Ionic mass fractions according to Niedermeier et al. (2008), Atlantic Ocean sample.
2007). Most VH-TDMA scans were performed on natural
SSA particles 71–77 nm in mobility diameter because this
was near the centre of the accumulation mode of SSA particles produced from our bubble generator, as measured by the
VH-TDMA in scanning mobility particle sizer (SMPS) mode
(Fig. 2). In addition scans were also performed for particles
towards the lower end (38 nm) and upper end (173 nm) of the
SSA accumulation mode.
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3
Results and discussion
3.1
3.1.1
VH-TDMA volatility curves
The organic fraction of 71–77 nm natural SSA
particles
71
77
71
Figure 3 displays the N71
0 , N0.03 , N0.08 and N0.3 volatility
curves as measured by the VH-TDMA. In this notation N
refers to natural SSA particles, the subscript number refers
to the sample water flow rate through the bubble generator
in Lmin−1 and the superscript number refers to the particle mobility diameter in nm (see Table 1). In addition the
A1 , A2 , A3 , NaCl1 and NaCl2 volatility curves are included
for comparison (the subscript number here is simply an index). The NaCl particles (square markers) were very stable
Atmos. Chem. Phys., 10, 2867–2877, 2010
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R. L. Modini et al.: Organics in Sea Spray Aerosol
N a C l1
N a C l2
A
A
1
2
A
3
N
7 1
N
0
7 7
N
0 .0 3
7 1
0 .0 8
N
7 1
0 .3
1 .1
1 .0
0 .9
V /V o
0 .8
1 .0 0
0 .7
0 .9 5
0 .9 0
V /V o
0 .6
0 .8 5
0 .8 0
0 .5
0 .7 5
1 0 0
2 0 0
3 0 0
4 0 0
5 0 0
0
T e m p e ra tu re ( C )
0 .4
0
5 0
1 0 0
1 5 0
2 0 0
2 5 0
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
nent not present in artificial SSA. We contend that in the temperature range 170–200 ◦ C the organic component of natural
SSA evaporated in the VH-TDMA. Beyond this temperature
the similarity of the volatility curves and point where complete evaporation begins (575 ◦ C) for natural and artificial
SSA is consistent with the explanation that they now have
similar, predominantly inorganic composition. The average difference between the artificial and natural SSA volatility curves in the temperature range 200–500 ◦ C can then be
taken as a measure of the average organic volume fraction of
the 71–77 nm natural SSA particles. This difference was 8%.
The standard deviation of the difference was 2% and the theoretical uncertainty was 6% (twice the V/Vo measurement
error). We take the larger value of ±6% as the absolute error
in the measured organic volume fraction of 8%.
0
T e m p e ra tu re ( C )
Fig. 3. Volatility curves of NaCl (squares), artificial SSA particles
(circles) and 71–77 nm natural SSA particles (diamonds) generated
using different water flow rates. Legend notation is described in text
and Table 1. Error bars represent ±3% measurement uncertainty in
V/Vo. Inset graph is magnified version of main graph with error
bars removed.
as volatilisation temperature was increased. A significant decrease in V/Vo was only observed at the highest temperature
obtained in these experiments, 583 ◦ C. This sudden decrease
indicated evaporation of NaCl had begun, which is consistent
with the onset temperature for particle formation in evaporation/condensation NaCl aerosol generation experiments
(Scheibel and Porstendörfer, 1983). The artificial SSA particles (circle markers) were more volatile than the pure NaCl
particles. Artificial SSA V/Vo decreased fairly steadily as
temperature increased so that only 82–83% of particle volume remained at 520 ◦ C. In contrast 96% of NaCl particle
volume remained at this temperature. The increased volatility of artificial SSA compared to NaCl particles could be because the evaporation or melting point of the mixture of inorganic salts was lower than the equivalent point for any pure
salt in that mixture. The volatility curves of all three artificial SSA experiments agreed within the V/Vo measurement
uncertainty. This indicates that small differences in the size
and inorganic composition of artificial SSA does not translate
into significant changes in the volatility curves, which confirms assumption number 3 of our VH-TDMA measurement
approach.
The 71–77 nm natural SSA particles (diamond markers)
were even more volatile than the artificial SSA particles. The
natural SSA volatility curves began to diverge from the artificial SSA curves at 170 ◦ C (see inset, Fig. 3). A small step
in the volatilisation curves was observed before they began
to level out at 200◦ C. Beyond this temperature the volatility
curves of natural and artificial SSA became very similar in
shape and appear almost parallel. The natural SSA particles
were expected to contain a seawater-derived organic compoAtmos. Chem. Phys., 10, 2867–2877, 2010
3.1.2
Dependence of organic fraction on water flow rate
through the bubble generator
The volatility curves of 71–77 nm natural SSA particles generated under varying water flow rates agreed almost completely within measurement uncertainty. That is, the organic fraction of 71–77 nm natural SSA particles produced
in our generator did not depend on water flow rate. If anything, the N71
0 curve lies slightly below all others. This indicates that the rate of transfer of organic material out of our
bubble generator by SSA is not sufficient to deplete the organic content of a 200 mL sample of natural SW, even after it has been bubbling at 100 mL min−1 for 24 h. This is
consistent with calculations of the amount of organic material exported by SSA from seawater in the bubble generator. As well as the measured accumulation mode (see
Fig. 2) there should have been a second super-micrometer
mode in our SSA size distribution (e.g. Keene et al., 2007;
Martensson et al., 2003). Therefore, we assumed a bi-modal
distribution with certain properties (accumulation mode:
median diameter=0.08 µm, concentration=10 000 cm−3 , organic mass fraction=80% ; super-micrometer mode: median
diameter=4 µm, concentration=200 cm−3 , organic mass
fraction=10%) and an organic density of 1.1 g cm−3 (Keene
et al., 2007). Although we didn’t measure such large organic
fractions in this study we purposely overestimated them for
this calculation. Under these assumptions and the conditions of our bubbling experiments only 2×10−4 g day−1 of
organic material would be exported by SSA from our sample water. If we assume the organic content of our 200 mL
sample of seawater was only 1 mg L−1 , it would take 23 h of
SSA generation to deplete all the organics in the water. If we
assume the seawater organic concentration was 10 mg L−1 ,
full depletion would take 234 h. The fact that organics were
not depleted in our generator after bubbling for 24 h suggests
that the organic content of our sample water was greater than
1 mg L−1 , or that we have overestimated the number and organic fraction of super-micrometer SSA particles in this calculation.
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R. L. Modini et al.: Organics in Sea Spray Aerosol
N
3 8
N
0 .0 8
7 1
N
0 .0 8
2873
N a C l H G F
1 7 3
A rt S S A H G F
9 0 %
0 .0 8
N a t S S A H G F
9 0 %
Z S R
9 0 %
H G F
9 0 %
2 .5 0
1 .0
2 .4 5
0 .9
2 .3 5
H G F
V /V o
9 0 %
2 .4 0
0 .8
2 .3 0
2 .2 5
0 .7
2 .2 0
0
1 5 0
2 0 0
2 5 0
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
0
0
5 0
1 0 0
1 5 0
2 0 0
2 5 0
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
0
T e m p e ra tu re ( C )
Fig. 4. Volatility curves of 38 nm (upside down triangles), 71 nm
(diamonds) and 173 nm (squares) natural SSA particles. Legend
notation is described in text and Table 1. Error bars represent ±3%
measurement uncertainty in V/Vo.
Dependence of organic fraction on particle size in
the range 38–173 nm
(N38
0.08 ),
(N71
0.08 ),
Figure 4 displays the 38 nm
and
71 nm
173 nm (N173
)
volatility
curves.
It
appears
that
volatility
0.08
increased slightly with decreasing particle size. In the temperature range 200–500 ◦ C where it is expected that the organic fraction of the particles has evaporated the average
difference (±1 standard deviation) between the 173 nm and
38 nm curves is 6±3%. The average difference between the
173 nm and 71 nm curves and the 71 nm and 38 nm curves in
the same range is 2±2% and 3±2%, respectively. However,
these volatility differences could be due to differences in particle size as well as composition (organic fraction). Therefore these values do not represent the difference in organic
volume fraction for the different particles sizes. Rather they
overestimate these fractions by an unknown amount equal
to the percentage change in V/Vo due to the change in initial particle size. Taking this into account and the fact that
the theoretical uncertainty in the calculation of organic volume fraction is 6%, we conclude that the organic fraction of
natural SSA particles did not vary significantly with particle
mobility diameter in the range 38–173 nm.
3.2
1 0 0
T e m p e ra tu re ( C )
0 .6
3.1.3
5 0
Hygroscopic growth factor measurements
To improve the representation of the shape- and Kelvincorrected bulk HGF90% data all of the measurements were
first categorised as NaCl, artificial SSA or natural SSA (71–
77 nm) particles. Then a number of measurements were
averaged at specific temperature values to obtain average
HGF90% for each particle type as a function of temperwww.atmos-chem-phys.net/10/2867/2010/
Fig. 5. Shape- and Kelvin-corrected bulk HGF90% values for
NaCl (squares), artificial SSA (circles) and natural SSA (diamonds)
particles as a function of volatilisation temperature. Each data
point represents an average of a number of measurements and error
bars represent ±1 standard deviation. Measurement uncertainty in
HGF90% was ±3%. Also included is the ZSR predicted HGF90%
curve (solid red line). See text for details on the calculation of this
curve.
ature. These averages are plotted in Fig. 5. At ambient
temperature NaCl HGF90% was 2.46±0.02. Artificial SSA
HGF90% was 4.4% lower at 2.35±0.02. These values both
agree well with theoretical predictions of NaCl and artificial
SSA HGF90% (e.g. Ming and Russell, 2001). Natural SSA
HGF90% was 3.9% lower than artificial SSA (8.1% lower
than NaCl) at 2.26±0.02.
It is instructive to observe how HGF90% for each particle
type varied as a function of temperature. NaCl HGF90% was
fairly constant until particle evaporation began at the highest
temperatures. At this point NaCl HGF90% decreased. Artificial SSA HGF90% continually increased with increasing
volatilisation temperature. Natural SSA HGF90% was below artificial SSA HGF90% up to a volatilisation temperature of 170 ◦ C. At this temperature the organic component
of the natural SSA particles began evaporating (see Fig. 3).
Coinciding with this the HGF90% curve started approaching
the artificial SSA HGF90% curve. At temperatures above
206 ◦ C the artificial and natural HGF90% curves agreed almost completely within experimental variation. The shapes
of the two curves were even very similar.
Under the ZSR approximation (Chen et al., 1973; Stokes
and Robinson, 1966) it is possible to investigate whether the
difference in the natural and artificial SSA HGF90% curves
at lower temperatures is consistent with the volume fraction
of organics in the natural SSA particles as calculated from the
volatility data (see Sect. 3.1). When making the ZSR approximation it is assumed that the individual components of an
internally mixed particle do not interact with each other and
therefore they uptake water independently. In practice this
Atmos. Chem. Phys., 10, 2867–2877, 2010
2874
R. L. Modini et al.: Organics in Sea Spray Aerosol
means that the HGF of a mixed particle can be calculated by
the volume-fraction-weighted sum of the HGF’s of individual components in that particle. We can use this assumption
to predict HGF90% for our natural SSA particles assuming
they are a binary mixture of an organic and inorganic (sea
salt) component. For input into the ZSR approximation we
use our measured organic volume fraction as a function of
temperature, our measured bulk HGF90% of artificial SSA
as a function of temperature and assume a bulk HGF90% for
the organic component of 1. This leads to a ZSR predicted
HGF90% curve which is plotted in Fig. 5. There is generally
good agreement between the ZSR predicted and measured
natural SSA HGF90% curves. At temperatures less than
200 ◦ C the ZSR predicted curve only slightly overestimates
the measurements. At temperatures greater than 200◦ C the
ZSR predicted HGF90% curve equals the artificial HGF90%
curve because it is assumed that all organics have evaporated
from the natural SSA particles and organic volume fraction
is set to 0 (Fig. 3).
In summary, the HGF90% data are consistent with the conclusions drawn from the volatility data. Namely, that our natural SSA particles had a minor organic component that evaporated over the temperature range 170–200 ◦ C. After evaporation the natural and artificial SSA particles had similar,
predominantly inorganic composition.
3.3
Implications
We have measured an organic volume fraction of 8±6% for
71–77 nm natural SSA particles that were generated from
samples of coastal seawater that most likely had high organic content. Note that this means aerosol-phase organics
were still enriched by tens or hundreds of times relative to
the sample water, depending on the exact concentration of
organics in the sample water. Assuming an organic density
of 1.1 gcm−3 (Keene et al., 2007) our measurement corresponds to an organic mass fraction of only 4%. We also
investigated the organic fractions of 38 and 173 nm natural
SSA particles and found these did not differ significantly
from the organic fraction of 71–77 nm particles. In comparison, Keene et al. measured an organic mass fraction of 80%
for 130 nm (GMD) natural SSA particles in similar bubbling
experiments. Facchini et al. measured an organic fraction of
77±5% for 125–250 nm natural SSA particles. These findings have led to the expectation that accumulation mode SSA
particles generated from biologically active or organic-rich
seawaters are predominantly organic. Our results suggest
that this is not always the case.
Hygroscopicity measurements in the literature also provide indirect evidence that accumulation mode SSA particles often contain only a minor organic fraction. Sea salt
aerosol (i.e. purely inorganic) is very hygroscopic. If a major, non-hygroscopic organic fraction is present in SSA it
will significantly decrease the hygroscopicity of that aerosol.
For example an SSA particle consisting of 20% sea salt
Atmos. Chem. Phys., 10, 2867–2877, 2010
(HGF90%=2.35) and 80% organics (HGF90%=1) will have
HGF90%=1.5 according to the ZSR approximation. A few
studies have reported HGF’s above deliquescence RH for accumulation mode natural SSA particles that are only slightly
below (<10%) corresponding NaCl or sea salt HGF’s (Niedermeier et al., 2008; Sellegri et al., 2008; Nilsson (2007)
referenced in Switelicki et al., 2008). This suggests the natural SSA particles investigated in these studies did not contain large organic fractions. In a very recent study Herich
et al. (2009) detected an organic component in both fresh
and aged 260 nm SSA particles at a remote continental site in
the arctic circle in northern Sweden. The authors found that
SSA organic content did not correlate with SSA hygroscopicity. This implies that the organic component only formed
a very minor fraction of total SSA mass, because changes in
the amount of organics present had no effect on particle hygroscopicity. Although these studies do not report measurements (e.g. organic content, biological activity) of the source
water from which aerosols were generated, they nevertheless suggest that accumulation mode SSA frequently contains only a minor organic fraction.
Discrepancies between the different studies could be related to not only the amount of organics present in the source
waters used in each experiment, but also the composition
and surface-active nature of those organics. For example
Facchini et al. (2008) conducted their measurements with
organic-rich Atlantic Ocean water collected during a plankton bloom. The Keene et al. (2007) results were obtained
using seawater collected from a Bermuda passage (Ferry
Reach). Measurements indicated the water was representative of the surrounding oligotrophic open ocean surface water. Our measurements were conducted with organic-rich
seawater, but this time collected at a coastal site significantly
affected by terrestrial run-off. The difference in the composition of organics (anthropogenic or biogenic, coastal or open
ocean) may affect how enriched they become in the aerosol
phase. We are not aware of any studies that have examined
the relationship between seawater type and SSA organic fraction. Therefore it is not clear what effect, if any, the use of
coastal as opposed to ocean seawater had on the SSA organic
fraction measured in this study.
In addition to seawater type, methodological differences
could have potentially contributed to some of the difference
between the results of this study and the results of the Keene
et al. and Facchini et al. studies. The two previous studies were conducted with freshly-collected seawater while in
this study seawater was stored in a dark refrigerator for up
to 2 weeks. The properties of seawater organics may have
changed during this storage time which could have affected
their transfer to the aerosol phase. However, we note that
no significant differences were observed between scans completed at different times during the 2 week measurement period. Therefore any artefacts due to storage are likely to be
minimal. The bubble generator employed in this study was
also far smaller than those used in previous studies (see water
www.atmos-chem-phys.net/10/2867/2010/
R. L. Modini et al.: Organics in Sea Spray Aerosol
volumes in Table 1). While this reduced the risk of external
organic contamination, it also meant our generator had a high
surface to volume ratio. As seawater flowed bottom-to-top
in the generator organics potentially adsorbed to the walls
thereby reducing the amount of organics eventually transferred to the aerosol. This potential loss mechanism was
not quantified or estimated. We do not believe that these
methodological differences can account for the order of magnitude difference between our measured accumulation mode
SSA organic fraction and the fractions measured in the Keene
et al. and Facchini et al. studies.
O’Dowd et al. (2008) have developed a combined organicinorganic sub-micron sea spray source function for modelling purposes. One input into this source function is the
organic mass fraction of sub-micron SSA as a function of
chl-a concentration, which was derived from ambient measurements conducted at Mace Head, Ireland. This function
saturates at 90% organic mass fraction for chl-a concentrations above 1.27 mg m−3 . Chl-a concentration was at least
this high, and probably higher, in the seawater used in this
study. However, the organic mass fraction of accumulation
mode SSA was only 4%. This suggests that the sub-micron
SSA organic fractions predicted by the O’Dowd et al. source
function may be significant overestimates of the true values
in regions other than off the coast of Mace Head.
These considerations point to the need for further independent, size-resolved measurements of the organic fraction of
SSA produced from a variety of different seawaters. Based
on the conflicting studies, it seems that there may be some
additional properties of seawater (e.g. organic composition,
surface-active nature of organics) that control how much organic material is transported from water to the aerosol phase
during the bubble bursting process. In addition, our results
suggest that if these experiments are conducted with bubble
generators, it may not be necessary to cycle water through
the generator to maintain a fresh supply of seawater-organics.
Bubble-generated SSA did not deplete the organic content of
static seawater in our bubble generator over a 24 h period.
4
Conclusions
A bubble generator was constructed and used to produce SSA
particles from samples of coastal seawater collected from
Moreton Bay on the east coast of Australia. Chlorophyll-a
measurements conducted close to the sampling point on either side of the collection date indicate that the samples were
biologically active and had high organic content. A VHTDMA was used to investigate the organic fraction of accumulation mode SSA particles produced from bubbling the
seawater in the generator. The volatility and hygroscopic
data collected by the VH-TDMA were in good agreement
with each other, and suggested that the natural SSA particles contained an organic component that evaporated in
the range 170–200 ◦ C. A comparison between the volatilwww.atmos-chem-phys.net/10/2867/2010/
2875
ity curves of artificial and natural SSA particles was used
to quantify the organic fraction of 71–77 nm natural SSA
particles at 8±6%. This measurement is an order of magnitude below comparable previous measurements of the organic fraction of accumulation mode SSA. At room temperature we measured shape- and Kelvin-corrected growth factors at 90% RH of 2.46±0.02 for NaCl, 2.35±0.02 for artifical SSA and 2.26±0.02 for natural SSA particles. We
reiterate that our results apply to accumulation mode SSA
generated from coastal as opposed to open ocean seawater.
Acknowledgements. This work was funded by the International
Laboratory for Air Quality and Health. The authors gratefully
acknowledge the South East Queensland Healthy Waterways
Partnership (www.healthywaterways.org) for provision of the
chlorophyll a data for Moreton Bay.
Edited by: A. Nenes
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